Blast furnace control method



Sept. 12, 1967 K H. GEE

BLAST FURNACE- CONTROL METHOD Filed March 31, 1964 5 TONE C/IHRGE l/VL55 JUL PHI/R IRO/V TEMPERATURE 5 Sheets-Sheet 1 IZOOO loaoo CAST I l ll l IZHM 3 6 INVENTOR fIenncf/I b. Gee

Sept. 12, 1967 K. H. GEE 3,341,323

BLAST FURNACE CONTROL METHOD Filed March 31 1964 3 Sheets-Sheet 2.STO/VE 45,7 5 000 Aeo E ll l ' INVENTOR Aennefh Gee United StatesPatent Ofifice 3,341,323 BLAST FURNACE CONTROL METHOD Kenneth H. Gee,Bethlehem, Pa., assignor, by mesne assignments, to Bethlehem SteelCorporation, a corporation of Delaware Filed Mar. 31, 1964, Ser. No.356,112 2 Claims. (Cl. 75-41) This invention relates to an improvedmethod of controlling the operation of a metallurgical blast furnace andmore particularly to a method of controlling the operation of anironmaking blast furnace based upon variations in slag basicity whichoccur from cast to cast.

Control of iron quality by blast furnace operators has been largelydependent upon interpretation of the appearance of slag, iron andtuyeres and upon analysis of silicon and sulfur in the iron. Suchmethods of control are unreliable.

In a copending application, Ser. No. 213,820, filed July 31, 1962, andnow abandoned, there is described a method of blast furnace controlusing iron temperature, as determined during a cast, as a basis foradjustment of the thermal balance of the furnace to maintain the furnacetemperature within proper range.

It is an object of this invention to provide a method of controlling theoperation of a blast furnace, which, together with the iron temperaturemethod of control described in the above copending application, willpromote more consistent operation than has heretofore been possible.

It is another object to control the operation of a blast furnace when itis performing within the desired iron temperature range by varying slagbasicity to meet the desired iron sulfur specification flux charged tothe furnace.

It is still another object of the invention to provide a method ofcontrolling the basicity of blast furnace slag in order to improvecontrol over the composition of the iron.

The objects and advantages of this invention will be more clearlyunderstood from the following description with reference to theaccompanying drawings in which below-average and then subsequentabove-average slag basicity determinations for a series of casts.

FIGURE 2 shows the application of the invention in the operation of afurnace which required a decrease and then an increase in stone chargeto compensate for initial above-average and then sub-sequentbelow-average slag basicity determinations for a series of casts.

the application of the invention in furnace which required an increasein stone charge to compensate for a gradually decreasing slag basicitywhich had the effect of causing an undesirably high iron sulfur.

One of the main functions of the blast furnace is to reduce the sulfurin iron. The reduction of sulfur is accomplished by flux materials. Thelime and magnesia in the flux combine with the gan gue in the ore burdenand the ash in the coke to form slag. Blast furnace slag compositionmust be controlled for good furnace operation. Adjustment of slagcomposition is normally accomplished by increasing or decreasing stonein the furnace charge.

Slag composition is commonly referred to in terms of slag basicity whichmay be expressed in terms of a basicity ratio. A common ratio and theone discussed hereinafter is the B/S ratio [(CaO+MgO)+SiO Other basicityratios could be used as well.

The ratio reflects the basicity of slag and increases as 3,341,323Patented Sept. 12, 1967 the basicity rises and decreases as slagbasicity falls. To increase the amount of sulfur removed from the ironthe ratio must be raised by increasing the flux charge (CaO-f-MgO). Ifmore sulfur can be tolerated in the iron the ratio may be reduced bydecreasing the flux charge. The amount of sulfur that has to be removedfrom iron varies from one operation to another and may dictate somewhatdifferent basicity ratios at various operations. Within a givenoperation however, sulfur removal from iron is controlled in the mannerdescribed above, i.e. by increasing or decreasing the slag basicityratio.

Flux is commonly charged as limestone or dolomite, generally referred toas stone, but it could also be charged through flux incorporated insinter. As mentioned above, the lime and magnesia in the flux combinewith the gangue in the ore burden and the ash in the coke to form slag.The gangue and ash generally have little chemical variation within aplant practice. Consequently, changing the flux charged to the furnacealters the slag basicity ratio in a predictable fashion.

Any satisfactory analytical method may be used for slag analysis, e.g.the quench method of analysis, which involves determining the liquidustemperature of a small sample. Slag basicities are normally reported tothe furnace operator within two hours after cast, along with ironsilicon and sulfur.

Variations in slag basicity occur during both flush and cast. Cast slaghas proved less variable than flush slag and is preferred for slagsampling. A standard time for sampling has been adopted, i.e. uponraising the second gate in the slag runner.

Decisions to alter the stone charge are based upon slag basicity, ironsulfur analysis, and the immersion thermocouple readings of irontemperatures described in the above mentioned copending application. Aslag basicity control range for each furnace is established from dataobtained from a given set of operating conditions. It is that range ofslag basicities which will produce iron sulfur within acceptable limitsfor a furnace operating within the desired iron temperaturegange. Forexample, a blast furnace was found to provide maximum production withminimum fuel costs when its maximum iron temperatures were kept withinthe range of 2720-2760 F. For this range of iron temperature, thefurnace operator was able to meet the desired specifications on ironsulfur, .035% average sulfur with no casts exceeding 0.050%, byadjusting slag basicity between 1.45 8/5 and 1.55 B/S by varying theamount of flux charged to the furnace.

In carrying out this invention, corrective stone changes are made whenslag basicity falls outside the control range for two successive castsprovided the iron temperature curves for those casts are normal. For anyparticular furnace there are periods when, for a given set of operatingconditions, the furnace is said to be operating smoothly or in balance.Throughout these periods a minimum of adjustment of the thermal balanceis required for operations, and iron production is high. Irontemperatures smooth operation, when the furnace is neither heating up orcooling down, will result in normal curves of similar appearance. Stonechanges are restricted to these periods of normal iron temperature.

In the accompanying figures, operating data are shown for maximumtemperature of iron, iron sulfur analysis, slag composition in terms ofthe B/S ratio,

and the pounds of stone per charge. The maximum temperature of iron is aconvenient guide and shows in a manner the thermal state of the furnacein these illustrations, and additional temperature data used by theoperator, such as temperature at appearance of slag, are not shownalthough developed during these periods. These additional data wouldnormally be shown in discussing the moves made to adjust the thermalstate of the furnace, as fully described in the aforementioned copendingapplication.

FIGURE 1 shows the above mentioned operating data for a series of sevencasts on a particular furnace. At the time of the casts the furnace wasoperating in a desired iron temperature range of 27102760 F. and theslag basicity control range had been established between 1.50 and 1.56in order to produce iron having a sulfur analysis of under .035 Asmentioned above, the slag basicity control range is established bymeasuring the basicity of the cast slag during a period when the furnaceis operating smoothly in the desired iron temperature range andproducing iron having a sulfur analysis at the maximum specified amountor less.

The iron sulfur analysis for casts 1 and 2 was undesirably above thespecified maximum of 035%. The slag basicity ratios for casts 1 and 2were well below the established range of between 1.50 B/S and 1.56 B/Swhile the iron temperatures for these casts were within the desired irontemperature range. Accordingly, the furnace operator increased theamount of stone on the furnace by 300 pounds per charge after cast 2 inan effort to reduce the iron sulfur. Iron temperature, iron sulfur andslag basicity were all within specified limits for cast 3. For casts 4and 5 iron temperatures were satisfactory and iron sulfur was wellwithin limits, but drifting lower. However, slag basicity had risenbeyond the desired control range during cast 4 and continued even higherfor east 5. In accordance with the method of the invention, the furnaceoperator, observing that the slag basicity was above the control rangefor two successive casts, reduced the stone charge 300 pounds. Irontemperature was within limits for east 6, sulfur was below the maximumallowable but slightly higher than the previous cast, and slag basicitywas still beyond the desired control range but coming down. By cast 7,iron temperature, iron sulfur and slag basicity were all within desiredlimits.

FIGURE 2 shows operating data for a series of eight casts on anotherfurnace. This furnace, at the time, was operating within a desiredtemperature range of between 2710 and 2760 F., with a slag basicityrange established between 1.50 B/S and 1.56 B/S to produce iron having asulfur analysis below .030%. After the furnace operator observed thatslag basicity exceeded the control range for two successive casts, 2 and3, and that iron temperatures were within specified limits, he decreasedthe amount of stone in the charge by 300 pounds. During casts 4 and 5slag basicity was within the control range but drifting downwardly, ironsulfur remained below 030% and iron temperatures varied only slightly.During casts 6 and 7 the downward trend of the slag basicity ratiocontinued and iron sulfur approached its upper limit of .030%. With irontemperatures within the desired operating ranges for these casts, thefurnace operator knew that slag basicity was the contributing cause tothe increasing iron sulfur. Accordingly, the stone charge was increased300 pounds. During cast 8 slag basicity increased sharply from the priorcasts and fell within the control range. Iron sulfur was satisfactoryand iron temperatures were within prescribed limits. Control factorsfell within specified limits for subsequent casts from this furnace.

FIGURE 3 shows a series of casts which again illustrate the value of themethod of this invention in controlling iron sulfur by slag basicitywhen iron temperatures are under control. The desired iron temperaturerange for the furnace from which these data were obtained was between2700 and 2750 F. It had been established that for this furnace toproduce iron bearing a sulfur analysis of under 040%, slag basicity hadto fall within the range of 1.50 B/S to 1.56 B/S, or, in another form,slag basicity could not deviate more than $0.03

from the desired level of 1.53 B/S. For cast 1 iron temperatures, ironsulfur and slag basicity were all within desired limits. For cast 2 theiron sulfur was at the upper limit of 040%, and slag basicity had fallento 1.49 B/ S, more than 0.03 below the desired level of 1.53 B/ S. Theiron temperatures for cast 3 were satisfactory but iron sulfur rosesharply to 050% and slag basicity for the second successive cast wasmore than 0.03 below the desired level of 1.53 B/S. Consequently, thefurnace operator increased the stone charge by 300 pounds to correct forthe slag basicity deficiency. Slag basicity and iron sulfur for east 4were both outside specified limits but iron temperatures, thoughdrifting lower, were still satisfactory. The stone change made aftercast 3 had not had time to affect furnace operation so no changes inoperation were made at this time. At cast 5 slag basicity was at thedesired level of 1.53 B/S, iron sulfur had dropped below the maximumspecified and iron temperatures were at 2740 F., well within limits. The300 pound stone change the furnace operator made after cast 3 hadproduced the desired results. Casts 6 and 7 found iron sulfur, slagbasicity and iron temperatures within specified ranges.

The data shown in the figures and described above were obtained fromfurnaces having a hearth diameter of 26' and an iron pr-oduction'ofbetween 2000 and 2200 tons per day. Stone changes for these furnaceswere made in increments of 300 pounds per charge. Naturally, it will beunderstood that the method of this invention can be practiced in anysize furnace regardless of the amount of the stone charge. The normal orstandard stone change must be sufiiciently large to be measuredaccurately by the furnace weighing system and should also be largeenough to cause a detectable change in the chemistry of the slag. On theother hand, a normal stone change should not be so large as to cause theslag composition to move from the lower to the upper edge of the controlrange. The 300 pounds stone change made in the above described examplesmet these requirements.

The slag basicity control aim is determined by the basicity whichprovides the desired iron sulfur when iron temperature is under control.Maintaining the furnace temperature and basicity of the slag withinproper range contributes to smoother furnace operation and results inimproved quality iron and increased tonnage.

It must be emphasized that for most satisfactory performance the slagbasicity method of controlling a blast furnace should be utilized onlywhen a furnace is operating smoothly and the iron temperatures, arewithin the established range of satisfactory performance. The slagbasicity control range or desired slag basicity is established fromoperating data as that range of slag basicities or that desired slagbasicity which will produce iron sulfur within acceptable limits forfurnaces; it is determined from experience during a period when thefurnace is operating smoothly.

Although certain novel features of my invention has been shown anddescribed, it will be understood that changes and modifications can bemade in the procedure without departing from the spirit of the inventionor the scope of the appended claims.

I claim:

1. The method of controlling the operation of a blast furnace to producemolten iron, having a sulphur content within a desired control range,from a burden comprising coke, ore and flux, which comprises:

(A) obtaining a slag basicity control range by (1) taking samples ofslag from a plurality of casts from said furnace when operating duringperiods when the iron temperatures 'as measured during casting fallwithin a desired temperature range and when producing iron having asulphur content within said desired control range,

(2) analyzing said slag samples to obtain the range of slag basicitiesthereof,

(B) obtaining the basicity of slag cast from said furnace duringsubsequent casts by (1) taking samples of slag from consecutivesubsequent casts, (2) analyzing said slag samples to obtain the slagbasicities thereof, and

(C) changing said burden when the iron temperatures as measured duringsubsequent casts fall within said temperature range by increasing theflux charge when the slag basicities of at least two of said consecutivesubsequent casts fall below said control range and by decreasing theflux charge when the slag basicities of at least two of said consecutivesubsequent casts fall above said control range.

2. The method of controlling the operati-on of a blast furnace toproduce molten iron, having a sulphur content within a desired controlrange, from a burden comprising coke, ore and flux which comprises:

(A) obtaining a slag basicity control range by (1) taking samples ofslag from a plurality of casts from said furnace when operating duringperiods when the iron temperatures as measured during casting fallwithin a desired temperature range and when producing iron having asulphur content Within said desired control range,

(2) analyzing said slag samples to obtain the range of slag basicitiesthereof,

(B) obtaining the basicity of slag cast from said furnace duringsubsequent casts by (1) taking samples of slag from two consecutivesubsequent casts, (2) analyzing said slag samples to obtain the slagbasi-cities thereof, and (C) changing said burden when the irontemperatures as measured during subsequent casts fall within saidtemperature range by increasing the flux charge when the slag basicitiesof said two consecutive subsequent casts fall below said control rangeand by decreasing the flux charge when the slag basicities of said twoconsecutive subsequent casts fall above said control range.

References Cited UNITED STATES PATENTS 2,208,245 7/ 1940 Boynton 75-412,832,682 4/1958 Reygagne 75-41 2,918,365 12/1959 Kanamori et al 75-413,030,150 2/1962 Reed 75-41 OTHER REFERENCES Bray-Ferrous ProductionMetallurgy, John Wiley & Sons, 1942, pages 164, 165.

The Making, Shaping and Treating of Steel, 7th ed.,

25 1957, page 258.

DAVID L. RECK, Primary Examiner. H. W. TARRING, Assistant Examiner.

1. THE METHOD OF CONTROLLING THE OPERATION OF A BLAST FURNACE TO PRODUCEMOLTEN IRON, HAVING A SULPHUR CONTENT WITHIN A DESIRED CONTROL RANGE,FROM A BURDEN COMPRISING COKE, ORE AND FLUX, WHICH COMPRISES: (A)OBTAINING A SLAG BASICITY CONTROL RANGE BY (1) TAKING SAMPLES OF SLAGFROM A PLURALITY OF CASTS FROM SAID FURNACE WHEN OPERATING DURINGPERIODS WHEN THE IRON TEMPERATURES AS MEASURED DURING CASTING FALLWITHIN A DESIRED TEMPERATURE RANGE AND WHEN PRODUCING IRON HAVING ASULPHUR CONTENT WITHIN SAID DESIRED CONTROL RANGE, (2) ANALYZING SAIDSLAG SAMPLES TO OBTAIN THE RANGE OF SLAG BASICITIES THEREOF, (B)OBTAINING THE BASICITY OF SLAG CAST FROM SAID FURNACE DURING SUBSEQUENTCASTS BY (1) TAKING SAMPLES OF SLAG FROM CONSECUTIVE SUBSEQUENT CASTS,(2) ANALYZING SAID SLAG SAMPLES TO OBTAIN THE SLAG BASICITIES THEREOF,AND (C) CHANGING SAID BURDEN WHEN THE IRON TEMPERATURES AS MEASUREDDURING SUBSEQUENT CASTS FALL WITHIN SAID TEMPERATURE RANGE BY INCREASINGTHE FLUX CHARGE WHEN THE SLAG BASICITIES OF AT LEAST TWO OF SAIDCONSECUTIVE SUBSEQUENT CASTS FALL BELOW SAID CONTROL RANGE AND BYDECREASING THE FLUX CHARGE WHEN THE SLAG BASICITIES OF AT LEAST TWO OFSAID CONSECUTIVE SUBSEQUENT CASTS FALL ABOVE SAID CONTROL RANGE.